Fuel cell devices are electrochemical devices, which enables production of electricity with high duty ratio in an environmentally friendly process. Fuel cell technology is considered as one of the most promising future energy production methods.
A fuel cell, as presented in FIG. 1, comprises an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. The reactants fed to the fuel cell devices undergo a process in which electrical energy and heat are produced as a result of an exothermal reaction.
In solid oxide fuel cells (SOFCs), oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with the used fuel 108 producing water and also, for example, carbon dioxide (CO2). Between the anode and cathode is an external electric circuit 111 for transferring electrons e− to the cathode. The external electric circuit 111 comprises a load 110.
FIG. 2 depicts an SOFC device, which can utilize as fuel for example natural gas, bio gas, methanol or other compounds containing hydrocarbons. SOFC device in FIG. 2 comprises planar-like fuel cells in stack formation 103 (SOFC stack). Each fuel cell comprises an anode 100 and cathode 102 structure as presented in FIG. 1. Part of the used fuel is recirculated in feedback arrangement 109 through each anode.
The SOFC device in FIG. 2 comprises a fuel heat exchanger 105 and a reformer 107. Heat exchangers are used for controlling thermal conditions in fuel cell process and more than one of them can be located in different locations of an SOFC device. The extra thermal energy in circulating gas is recovered in the heat exchanger 105 to be utilized in SOFC device or outside in a heat recovering unit. The heat recovering heat exchanger can thus be located in different locations than that presented in FIG. 2. The reformer is a device that converts the fuel such as, for example, natural gas to a composition suitable for fuel cells, for example, to a composition containing half hydrogen and other half methane, carbon dioxide and inert gases. The reformer is not, however, necessary in all fuel cell implementations, because untreated fuel may also be fed directly to the fuel cells 103.
Measurement means 115 (such as fuel flow meter, current meter and temperature meter) can be used to carried out measurements for the operation of the SOFC device from the through anode recirculating gas. Only part of the gas used at anodes 100 (FIG. 1) of the fuel cells 103 is recirculated through anodes in feedback arrangement 109. FIG. 2 depicts diagrammatically another part of the gas being exhausted 114.
Heat management of the fuel cell stacks is one of the key functions of the balance of plant (BoP) equipment in a high temperature fuel cell system. The heat balance of the fuel cells stacks is affected by many mechanisms including internal reforming, fuel cell reactions, heat transport by flow of reactants and direct heat exchange with the surrounding structures. Exemplary methods for the control of the temperature balance comprise adjustment of the internal reforming rate and adjustment of air flow and cathode inlet temperature.
System heaters can be implemented as electrical heaters comprising heating resistors or gas burners or a combination thereof. Benefits of electrical heaters include excellent controllability and the ability to place them directly in reactant streams or structures to be heated. A drawback of electrical heaters is their tendency to generate earth fault currents, particularly in the case of high temperature heaters. Also fuel cell stacks, being high temperature electrical devices, may generate significant earth fault currents depending on isolation arrangements in their reactant feed, supporting structures and loading arrangement.
High temperature fuel cells (MCFC, SOFC) have operating temperatures in the range of 600-1000° C. High temperatures are used to achieve sufficient conductivity for proper operation, i.e., sufficiently low area specific resistance (ASR) to draw currents from the fuel cells. Below their nominal operating temperature, these fuel cells experience an increased ASR, which limits the amount of current that can be drawn from the cells. Since heat production in the cells is proportional to the current, sufficient heat production within the fuel cell for maintaining an operating temperature is not achievable until at relatively high currents. For example, 50% of nominal current or higher can be required before the heat production within the stack suffices to compensate for mechanisms of heat removal. These mechanisms for heat removal include endothermic reactions taking place at fuel cell anodes (internal reforming), heat transport by reactant flow and heat transfer to surroundings.
As a consequence of the inability of stacks to internally heat up at low temperatures, heat is applied from an external source in system start-up until fuel cells are relatively close to their nominal operating temperature, allowing for currents large enough to bring the heating further. A convenient method for applying external heat to fuel cell stacks during start-up is to utilize electrical heaters offering excellent controllability and flexibility with respect to placement at a moderate price. The price and complexity of electrical heaters is in turn strongly related to the maximum temperature and heat duty at which the heaters are required to operate. Hence, the last tens of degrees required from the electrical heaters in order to bring the fuel cells up to near-nominal temperatures can determine their dimensions. A small reduction in the maximum temperature that needs to be achieved or in the heat duty that needs to be delivered at this temperature would have a significant effect on system compactness and on heater sizing, thus also reducing economical costs.
For example, a significant mechanism for heat removal inside stacks is internal steam reforming in which methane reacts with steam, i.e., H2O, to hydrogen and carbon monoxide in a strongly endothermic process:CH4+H2O=CO+3H2 
During nominal operation of the fuel cell, this mechanism of heat removal is beneficial in reducing the need for stack cooling by other means e.g., excessive air feed. In a system, the internal reforming may account for up to 75% or more of the heat removal from stack reactions. During system heat-up, the cooling effect of internal reforming is, in turn, clearly a negative effect as it increases the amount of heat that needs to be applied from external sources, e.g., electrical heaters. The amount of internal reforming depends on the fuel feed and the extent of reforming that takes place outside the stacks, e.g., in a pre-reformer. In the absence of a pre-reformer or in the case of an adiabatic pre-reformer, most of the internal reforming will take place in the stack thus providing a significant cooling effect proportional to the fuel feed. The fuel feed is in turn substantially proportional to the current as system fuel utilization is better to be maintained in a narrow range, for example, 75-85%.
Another problem is that fuel cell voltage does not sink linearly as a function of loading current. In a starting stage of the loading, voltage is much higher than in nominal loading conditions. For example, it can be beneficial for power electronics to be designed to withstand high voltage in the start up. This can lead to non-optimal component selection because higher voltage rated components suffers from considerably higher power losses in nominal loading conditions.